Dipole moments of partially bound lewis acid - base adducts

Stark effect measurements have been performed on six Lewis acid-base complexes containing ³²SO₃ and ¹¹BF₃. The following dipole moments have been obtained: HC¹⁴N-SO₃ (4.4172 ± 0.0031 D); CH₃C¹⁴N-SO₃ (6.065 ± 0.018 D); HC¹⁵N-BF₃ (4.1350 ± 0.0073 D); H₃¹⁵N-BF₃ (5.9027 ± 0.0093 D (A state), 5.917 ± 0.010 (E state)); (CH₃)₃¹⁵N-BF₃ (6.0157 ± 0.0076 D); and (CH₃)₃¹⁵N-B(CH₃)₃ (4.5591 ± 0.0097 D). Across a series of complexes with a common acid, the induced dipole moment increases sharply as the dative bond shortens. Contributions to the total molecular dipole moment arising from distortion, polarization, and charge transfer have been estimated for these and a number of related complexes, using the block-localized wave function energy decomposition analysis of Mo, Gao, and Peyerimhoff. Mulliken and natural population analyses are presented, as are electron density difference maps for HCN-SO₃, (CH₃)₃N-BF₃, and (CH₃)₃N-SO₃. Theoretical values for the degree of charge transfer are compared with experimental estimates based on nuclear hyperfine parameters, and the validity of a simple chemical model involving charge transfer and bond moments is examined. Ab initio calculations of the induced dipole moment of HCN-SO₃ and H₃N-SO₃ are given as a function of N-S bond length and compared with the experimentally observed values for a series of SO₃ complexes. The results suggest that the induced moments of the series collectively approximate the induced dipole moment function for individual members of the series. Similar results are obtained using previously published dipole moment functions for HCN-BF₃ and H₃N-BF₃.